8218966: AArch64: String.compareTo() can read memory after string

[Dmitrij Pochepko]

Hi all,

Please review a fix for "8218966: AArch64: String.compareTo() can read 
memory after string".

bug: https://bugs.openjdk.java.net/browse/JDK-8218966
webrev: http://cr.openjdk.java.net/~dpochepk/8218966/webrev/

Intrinsic implementation returns wrong value in rare cases for strings 
longer than 72 characters.

Changes:

- Different encodings case. Small 16-characters loop and post-loop code 
are re-organized to stop at string end. Post-loop now also uses 
compare_string_16_x_LU() to avoid code duplication.
- Changed calculation of prefetchLoopExitCondition. It might be 
incorrect in case when SoftwarePrefetchHintDistance was set to 
non-default small value.
- Same encoding case. Moved loop counter update out of prefetch block. 
It might miss end-of-string check when prefetch is disabled, with memory 
after string being read.
- Added 2 tests. They are quite similar but 
TestStringCompareToSameLength compares strings of same length, and 
TestStringCompareToDifferentLength is for different lengths. Tests cover 
8218966 case. And also they cover different parts of intrinsic, taking 
into account conditions in the implementation and possible 
SoftwarePrefetchHintDistance values.

Testing:

Existing jtreg and jck tests were not able to detect 8218966 case. But 
they pass with the fix applied. Newly added jtreg tests can detect the 
issue and potential problems in case of changes in the implementation. 
The following testing was performed:

- jck with default vm flags
- jck with -Xcomp -XX:-TieredCompilation
- hotspot jtreg tests (including new tests): compiler/*, runtime/*, gc/* 
with default vm flags
- hotspot jtreg tests (including new tests): compiler/*, runtime/*, gc/* 
with -Xcomp -XX:-TieredCompilation
- jdk jtreg tier1-3 tests with default vm flags
- jdk jtreg tier1-3 tests with -Xcomp -XX:-TieredCompilation

No regressions were found.

I'd like to thank Pengfei Li (Pengfei.Li at arm.com) for pre-review and 
additional testing.

I'm also about to send separate additional webrev with compareTo 
intrinsic documentation and maintenance-related improvements as separate 
enhancement.

Thanks,
Dmitrij